As shown in FIG. 1, a cellular radio communications system 100 is mainly composed of terminals 101, base stations and a core network 103. The network composed of base stations is called a Radio Access Network (RAN), which is responsible for non-access stratum transactions, such as radio resource management. Physical or logical connections may exist between base stations according to practical situations, for example, between base station 102A and base station 102B or base station 102C as shown in FIG. 1. Every base station can be connected with one or more than one core network 103 (CN) nodes. Each CN is responsible for non-access stratum affairs such as location update, etc., moreover, for the update of the anchor point on the user side. The terminal 101 are all sorts of devices that can communicate with cellular radio communications networks, such as a mobile telephone or a laptop computer, etc.
The cellular radio communications system 100 is identified with radio frames as the basic unit in terms of system time, and each radio frame is numbered with what is called a System Frame Number (SFN). The terminal 101 can get the start of the radio frame by cell search, thereby acquiring the downlink time synchronization. In cellular radio communications systems such as in a Universal Mobile Telecommunications System (UMTS) and a Long Term Evolution (LTE) system, the length of a radio frame is 10 milliseconds (ms). Different cellular radio communications systems may have different frame structures of radio frames, and a radio frame usually comprises an integer number of subframes. When getting the downlink synchronization, the terminal 101 may get to know the location of the current subframe within the current radio frame. As shown in FIG. 2, in an LTE system, a TYPE1 radio frame 200 comprises 10 subframes and each subframe comprises 2 time slots. Such a frame structure is usable for Frequency Division Duplex (FDD) and High Chip Rate Time Division Duplexing (HCR TDD); as shown in FIG. 3, a TYPE2 radio frame 300 comprises 2 subframes and each subframe comprises 7 time slots and an interval time slot between the first two time slots. Such a frame structure is applicable for Low Chip Rate Time Division Duplexing (LCR TDD). In some cellular radio communications systems the unit at a lower level in the radio frames is called a time slot. For example, in FDD system of Wide band Code Division Multiple Access (WCDMA), there are 15 time slots in a radio frame. The random access time slot as the term is used herein means the subframe or a time slot or a time slot in a subframe at a lower level in a radio frame of a cellular radio communications system 100. As to the TYPE1 radio frame in the LTE system, the random access time slot is usually a 1 ms-long subframe; as to the TYPE2 radio frame in LTE system, the random access time slot is usually a 5 ms-long time slot in a subframe. However, exceptions exist. For the purpose of broad coverage, the random access time slot in the LTE system may occupy 2 or 3 subframes or time slots, and these random access preambles are sometimes called extended pulse; while in LCR TDD, a kind of short random access preamble can be adopted in very small cells, and such preamble is shorter than common time slots. One random access time slot occupies a bandwidth of 6 radio Resource Blocks (RB) in a frequency domain.
In an existing LTE system, the procedure of a terminal randomly accessing to a cellular radio communications system comprises the following three steps:
a. a terminal sends a random access preamble message to a base station in a certain random access time slot in a radio frame;
b. the base station returns a random access response message to the terminal, terminal and the message comprises at least the uplink radio resource;
c. the terminal sends a message in the uplink radio resource allocated for the terminal by the base station.
In step a, it may happen that one or more than one terminals send random access preamble messages to the base station in the same random access time slot. These random access preamble messages may be different from one another, or the same with one another which means that the same random access preamble pseudo random code is adopted. The base station can identify the random access preamble messages adopting different pseudo random codes in the same random access time slot, but cannot identify the random access preamble messages adopting the same pseudo random code.
In step b, the random access response message may comprise response information responding to one or more than one random access preamble messages. These random access preamble messages are sent from the same random access time slot. The operation, combining possible more than one random access response messages in one response message, is mainly to increase the utilization rate of radio resources during the random access procedure. In order to identify the random access response message by the terminal the base station will add a temporary group identifier to the message, and a corresponding relation exists between the group identifier and the random access time slot. Meanwhile, the random access response message also comprises an individual identifier corresponding to the random access preamble message itself, which is usually the index number of the random access preamble in the random access preamble set in the cell in which the random access preamble is located. The method for setting a group identifier is predefined in the protocol. When a certain terminal sends a random access preamble message to the base station, the terminal already knows the expected values of the group identifier and the individual identifier in the random access response message that the terminal will receive.
In order to maintain certain flexibility of the random access response procedure, the random access response message is asynchronous with the random access preamble message in a time domain, i.e., there is no fixed relation between the two messages in the time domain; on the contrary, it is allowed to send random access response messages within one time window. Meanwhile, for increasing the flexibility of radio resource scheduling, the base station, in order to respond to random access preamble messages received in a certain random access time slot, can send random access response messages corresponding to the above-mentioned random access preamble messages at one or more Transmission Time Interval (TTI) within the above time window. A start time of the time window is related to how fast the base station processes the random access preamble messages, and an end time thereof is related to the load of the base station processing random access preamble messages, the radio resources scheduled to random access response messages and other factors.
In step c, after receiving one random access response message within the specified time window, the terminal verifies whether the message comprises the expected group identifier first, which is usually included in the physical control channel; if the message comprises the expected group identifier, the terminal verifies whether the message further comprises an individual identifier corresponding to the sent random access preamble message; if the expected individual identifier is included, it can be determined that current random access response message corresponds to the sent random access preamble message. Afterward, the terminal sends a message in the uplink radio resource allocated for the terminal by the base station in step b according to practical demand. The sent message could be a request of layer 3 for establishing radio connection, a switch response, a scheduling request, or an uplink synchronization request, etc.
In the existing published technology, methods of setting group identifiers in step b are provided. Generally, a group identifier can be computed according to the absolute locations of random access time slots in system time as well as those in the frequency domain in these methods, so that the group identifier is unique within a specified time period.